The present invention relates to an optical device which is used in an optical fiber communication system and, more particularly, to an optical device which is suitable for an information distribution optical network represented by a cable television and to a synthesizing method of such an optical device.
Communication networks have been supported so far by of electronic circuits techniques (reference literature: Minoru Akiyama, et al., "Digital Telephone Exchange", Sangyo Tosho Co., Ltd., published in 1986). In recent years, the development of optical fiber communication system which can transmit information to a long distance at a high speed is remarkable and the realization of such optical communication systems is progressing.
Among such systems information distribution type optical networks are advancing in such a direction as to distribute a larger amount of information to a larger number of terminals. In the case of optical cable television as an example, there is a tendency to increase that the number of channels and the number of subscribers increase. When the number of channels increases, it is necessary to widen a range of the signal selection. According to a method of selecting the channel in a receiver which the subscriber possesses, it is difficult to cope with an increase in number of channels. It is, therefore, necessary to use a method whereby, on the side which serves information, the channel selection is controlled and only the necessary signal is distributed to the subscriber in accordance with a request from the subscriber. A construction to realize such a method by present existing devices is as shown in FIG. 2. Signals 1-1 to 1-m of respective channels are distributed to selectors 3-1 to 3-n corresponding to subscribers (1 to n) by optical amplifying distributors 2-1 to 2-m. Distribution signals 4-1 to 4-n selected by the selectors 3-1 to 3-n in accordance with request signals 5-1 to 5-n are sent to the subscribers.
Each of the optical amplifying distributors 2-1 to 2-m can be realized by a combination of an optical amplifier and a photocoupler. As a selector of a photosignal, it is possible to apply an optical switch as described in the literature, "Journal of Lightwave Technology", Vol. 9, No. 7, pages 871-877, 1991.
According to the above conventional method, since an increase in number of subscribers or an increase in number of channels causes an increase in number of amplification distributions or an increase in number of channels which can be selected, there is a problem such that the scales of the photocoupler for separation and the optical switch and their connecting wires increase. Since the signals of the channels which are not selected are also amplified, there is also a problem such that an unnecessary electric power is consumed. There is further a problem such that it is necessary to exchange the optical switch to another optical switch having a larger number of inputs in association with an increase in number of channels and there is a problem of an expandability. According to the above conventional method, on the other hand, since it is necessary to use the amplification distribution and optical transmission path for the channel which is hardly selected, there is a problem such that a whole scale increases due to the parts which are hardly used.
Another conventional technique is entitled "Optical Switch Element" disclosed in, JP-A-61-91623. Such a technique relates to a method of loss modulating each signal after the signals are distributed toward all of the output terminals, so that there is a problem such that the signal which is not selected is abandoned. For instance, since a separation loss occurs at every optical switch, even if the light intensity is compensated by amplification, when the number of distributions increases, the S/N ratio deteriorates. Since the unnecessary signals are also distributed, an intensity which is distributed to the signal to be selected decreases and a total amplification amount and an electric power consumption increase. A further conventional technique is shown in Kishimoto et al., "Optical Self-Routing Switch Using Integrated Laser Diode Optical Switch", IEEE Journal on selected areas in communication, Vol. 6, No. 7, Aug., 1988.